Additive fabrication processes utilize computer-aided design (CAD) data of an object to build three-dimensional parts layer-by-layer. These three-dimensional parts may be formed from resins, powders, or other materials. Additive fabrication processes for producing three dimensional articles are known in the field.
An example of an additive fabrication process is stereolithography (SL). SL is a well-known process for rapidly producing models, prototypes, patterns, and production parts in certain applications. SL uses CAD data of an object wherein the data is transformed into thin cross-sections of a three-dimensional object. The data is loaded into a computer which controls a laser beam that traces the pattern of a cross section through a radiation curable resin composition contained in a vat, solidifying a thin layer of the resin corresponding to the cross section. The solidified layer is recoated with resin and the laser beam traces another cross section to harden another layer of resin on top of the previous layer. Optionally, the machine must dwell between the recoating step and the tracing of the next cross section to allow the radiation curable resin to equilibrate. The process is repeated layer by layer until the three-dimensional object is completed. When initially formed, the three-dimensional object is, in general, not fully cured and therefore may be subjected to post-curing, if required.
SL is a vat-based additive fabrication process. Vat-based systems consist of a large reservoir, or vat, of radiation curable resin wherein the imaging occurs. A vertically movable elevator platform is submersed in the vat and is used for supporting the solid three-dimensional object as it is built. The speed of the vat-based additive fabrication process is inhibited by, for instance, the recoating process and optional dwell time that must occur between building each layer of the three-dimensional object.
Additive fabrication systems have been developed wherein the imaging occurs using a vat-based system that is modified to use a foil or film to aid in forming each layer. Such a technique is disclosed in, for example, U.S. Pat. Nos. 5,171,490, 7,052,263, and 7,438,846.
Further additive fabrication systems have been developed that are known as vat-less systems. Vat-less systems differ from traditional vat-based systems in that the imaging step does not occur within the reservoir of radiation curable resin. Instead, the layer of radiation curable resin is transported to the imaging area one layer at a time. Examples of vat-less systems are disclosed in, for example, European Patent EP1710625 and U.S. Pat. Nos. 6,547,552, 7,614,866, 7,758,799, and 7,731,887. An example of a commercially available vat-less system is the V-FLASH® system available from 3D Systems, Inc. Some of these machines employ an upside down build platform wherein the part is translated vertically upwards as it is built rather than vertically downwards as in a traditional vat-based apparatus.
Systems that require a step of separating the just cured solid layer from a separating layer or carrier such as a film, foil, glass, or plate are referred to throughout this application as substrate-based additive fabrication processes. Such separating layers and carriers will be collectively referred to as substrates throughout this application.
Substrate-based processes commonly follow the same general method of coating a layer of radiation curable resin on a substrate, contacting the layer of radiation curable resin with a previously cured layer, exposing the radiation curable resin to actinic radiation thereby forming a cured layer which adheres to the previously cured layer, separating the cured layer and the substrate, and repeating the steps a sufficient number of times in order to build up a three-dimensional object.
Although the use of substrates in an additive fabrication process provides several improvements over traditional vat-based systems, the use of substrates also presents several challenges. For instance, the substrate-based process introduces the complexity of accurately and sufficiently coating the substrate with radiation curable resin. Insufficient or uneven coating or dewetting of the radiation curable resin from the substrate can negatively impact the building of a three-dimensional object in a substrate-based process. Furthermore, the increased speed of the process requires that proper green strength is developed in the freshly cured layer of resin in order to facilitate proper peeling of the freshly cured layer from the substrate and proper bonding to the previously cured layer.
Adhesion of the curing radiation curable resin to the substrate must be managed. A freshly cured layer of radiation curable resin develops both cohesive strength to the previously cured layer and adhesive strength to the substrate as the layer solidifies. It is critical that the freshly cured layer of radiation curable resin fully peels off of the substrate. The peeling off of a freshly cured layer of radiation curable resin from the substrate is known as adhesive failure.
The adhering of a freshly cured layer to the previously solidified layer is known as cohesive strength. Developing good cohesive strength to the previously cured layer is important for all additive fabrication applications. However, cohesive strength is greatly more important in substrate-based additive fabrication processes over additive fabrication processes that do not utilize a substrate because of the added forces caused by the separating of the substrate and the freshly cured layer of radiation curable resin. Cohesive failure occurs when the freshly cured layer of radiation curable resin adheres more to the substrate than to the previously cured layer and either does not fully separate from the substrate or causes some separation among the previously cured layers. In summary, the build will fail unless adhesive failure at the curing layer/substrate interface occurs before cohesive failure for each and every layer of the build.
It is well known in the field of radiation curable resins that hybrid radiation curable resins produce cured three-dimensional articles with the most desirable mechanical properties. A hybrid radiation curable resin is a radiation curable resin that comprises both free radical and cationic polymerizable components and photoinitiators. Generally, the rate of cationic polymerization in a radiation curable resin is considered too slow for additive fabrication applications unless a sufficient amount of free-radically polymerizable components are incorporated into the radiation curable resin. The rate of photoinitiated free-radical polymerization is very fast, much faster than the rate of photoinitiated cationic polymerization. Consequently, the mechanical properties of the cured three-dimensional article develop over time after the initial cure of the hybrid radiation curable resin. Additionally, hybrid radiation curable resins present challenges due to the slower rate of cure relative to radiation curable resins that are made of only free-radically polymerizable components, such as increased adhesion to the substrate. Perhaps consequently, the use of hybrid radiation curable resins in substrate-based additive fabrication processes is not well known. Hybrid radiation curable resin formulations have been preferred in additive fabrication applications that do not use a substrate.
Free-radically polymerizable components generally develop less adhesive strength to the substrate than cationically curable components. Perhaps consequently, commercial attempts at formulating liquid radiation curable resins for use in substrate-based additive fabrication processes have generally resulted in entirely free-radically based compositions. Such compositions generally contain a mixture of various (meth)acrylates and/or urethane(meth)acrylates. See U.S. Pat. No. 7,358,283 and WO2010/027931, both to 3D Systems, Inc, Urethane(meth)acrylates are widely known to be mostly incompatible with cationically cured systems. Please see Vabrik et al., “A Study of Epoxy Resin—Acrylated Polyurethane Semi-Interpenetrating Polymer Networks,” Journal of Applied Polymer Science, Vol. 68, 111-119 (1998).
Hybrid cure radiation curable resins have an increased adhesive strength to the substrate than do radiation curable resins composed of entirely free-radically curable components. Hybrid cure liquid radiation curable resins useful in a substrate based additive fabrication process and a process for substrate based additive fabrication are disclosed in, for example, PCT/US2010/60722, which is hereby incorporated by reference in its entirety.
In order to facilitate desirable peeling performance, many additive fabrication processes have utilized flexible substrates. By flexible, it is meant that the substrates are not rigid, such that the substrates can, for instance, be transported on a roll rather than in flat sheets. Flexible substrates may be elastic, as in U.S. Pat. No. 7,438,846, or substantially inelastic, as in U.S. Pat. No. 7,731,887, Specifically, flexible substrates are desirable in a wide variety of substrate-based additive fabrication processes.
Various types of flexible substrates that are useable in substrate-based additive fabrication processes are known in the art. For example, U.S. Pat. No. 5,171,490 to Fudim discusses a substrate-based additive fabrication process. Fudim prefers a fluorinated ethylene propylene copolymer. U.S. Pat. No. 5,447,822 to 3D Systems, Inc. discusses a similar device and mentions that substrates made of Teflon®, MYLAR, or epoxy are acceptable as long as they are UV-transparent and can have appropriate thickness.
U.S. Pat. No. 5,637,169 to 3D Systems, Inc. discusses an apparatus where a reactive resin is contained between a supporting and a protective sheet. This patent mentions a supporting sheet made of polyester and a protective sheet made of polyethylene or Teflon®.
Other publications that discuss the use of flexible substrates are US20060249884 to 3D Systems, Inc. which mentions polypropylene, polycarbonate, or polyethylene as possible substrate materials. U.S. Pat. No. 7,614,866 to 3D Systems, Inc. discusses substrates made of fluoropolymer resins, poly(propylene), poly(carbonate), fluorinated ethylene propylene, and mixtures and copolymers thereof. Polytetrafluoroethylene (PTFE) films are also mentioned as being useful.
WO2010/74566 to TNO discusses the use of silicone (applied as a coating to a foil of a different material) and TPX. KR10-06414 to Carima discusses the use of a modeling sheet, being glass or acryl, that may be coated with one or more of Teflon®, nylon, transparent fiber, and polyester to allow for easy separation of the modeling sheet.
As demonstrated above, various flexible substrates are known. However, the various known flexible substrates are not able to meet the various challenges of the advancements in substrate-based additive fabrication technologies. A flexible substrate that can better withstand the rigors of advanced substrate-based additive fabrication process would thus be highly desirable.